1. Field of the Invention
The present invention is concerned generally with a fluid-filled elastic mounting structure, and more particularly with a fluid-filled elastic mounting structure suitablely used for example as an automotive engine mount, which exhibits an improved damping characteristic with respect to low-frequency large-amplitude vibrations, and a sufficiently low dynamic spring rate or constant with respect to medium-frequency medium-amplitude to high-frequency small-amplitude vibrations.
2. Discussion of the Prior Art
A mounting device or structure for flexibly attaching an engine or engine unit of a motor vehicle to the body of the vehicle is generally required to exhibit excellent vibration damping characteristics for the low-frequency large-amplitude vibrations applied thereto, and at the same time provide an improved vibration isolating effect with respect to a comparatively wide range of vibration frequencies, i.e., medium-frequency medium-amplitude to high-frequency small-amplitude vibrations. In particular, such a mounting structure is required to exhibit an excellent damping effect when the low-frequency large-amplitude vibrations are applied thereto.
In an attempt to satisfy the above requirements, there has been proposed a fluid-filled elastic mounting structure including (a) first and second support members which are opposed and spaced apart from each other in load-receiving direction in which a vibrational load is applied to the mounting structure, (b) an elastic body interposed between the first and second support members, for elastically connecting these two support members, (c) a closure member which has a flexible portion and which is secured to the second support member, so as to cooperate with the elastic member to define therebetween a fluid chamber filled with a non-compressible fluid, (d) a partition assembly disposed within the fluid chamber, so as to divide the fluid chamber into a pressure-receiving chamber on the side of the elastic body, and an equilibrium chamber on the side of the closure member, and (e) an orifice for fluid communication between the pressure-receiving chamber and the equilibrium chamber.
The fluid-filled elastic mounting structure constructed as described above is capable of excellently damping low-frequency vibrations, based on resonance of the fluid mass flowing through the orifice, by tuning the orifice so that the resonance frequency of the fluid mass in the orifice is adjusted to a relatively low level corresponding to the frequency range of the vibrations to be damped.
While the tuning of the orifice enables the mounting structure to exhibit improved damping characteristics for the low-frequency vibrations, the vibration isolating capability of the mounting structure upon application of the medium to high-frequency vibrations is undesirably low, due to an increased dynamic spring rate of the structure because the non-compressible fluid is less likely to flow through the orifice.
In view of the above drawback, an improvement of the fluid-filled elastic mounting structure of the type discussed above is proposed according to laid-open Publication No. 57-9340 of unexamined Japanese Patent Application. This mounting structure employs a movable member which is disposed in the partition assembly between the pressure-receiving and equilibrium chambers. The movable member is deformable or displaceable over a suitable distance in the load-receiving direction, so that a pressure difference between the pressure-receiving and equilibrium chambers may be absorbed or accommodated by the oscillating deformation or displacement of the movable member, in order to avoid a rise in the pressure in the pressure-receiving chamber.
The deformation or displacement of the movable member which involves flows of the fluid is effective to reduce the amount of transmission of the input vibrations between the first and second support members, when the frequencies of the vibrations are in the neighborhood of the resonance frequency of the fluid. However, the dynamic spring constant of the mounting structure is extremely increased when the structure is subject to vibrations having frequencies higher than the resonance frequency of the fluid. Therefore, the resonance frequency of the fluid flows due to the oscillating displacement of the movable member is adjusted to a relatively high level, for the purpose of minimizing the amount of reduction in the vibration isolating function of the mounting structure which results from the increased dynamic spring constant. Although this arrangement enables the mounting structure to exhibit a sufficiently low dynamic spring constant for the vibrations having comparatively high frequencies around the resonance frequency of the fluid, the mounting structure is uncapable of providing a sufficient vibration isolating effect with respect to the medium-frequency vibrations.
Thus, the proposed fluid-filled elastic mounting structure having the movable member is not satisfactory in meeting the above-indicated requirements, particularly, in terms of its dynamic spring constant with respect to the medium-frequency vibrations. This problem is serious especially where the mounting structure is used as an engine mount for an automotive vehicle, which is required to provide not only a high damping effect for low-frequency large-amplitude vibrations such as engine shakes of 5-15 Hz, but also a high isolating effect for both medium-frequency medium-amplitude vibrations such as engine-idling vibrations of 20-30 Hz and high-frequency small-amplitude vibrations such as booming noises of 100-300 Hz.